1. Field of the Invention
This invention resides in the field of nanocrystalline materials and processes for their manufacture.
2. Description of the Prior Art
Quantum-sized particles, i.e., those having diameters within the range of about 0.1 nm to about 50 nm, also known as quantum dots or nanocrystals, are known for the unique properties that they possess as a result of both their small size and their high surface area. Some of these particles have unique magnetic properties that make the particles useful in ferro fluids, in magnetic tagging elements, and in electronic data systems such as recording media. Luminescent nanocrystals are particularly useful as detectable labels such as oligonucleotide tags, tissue imaging stains, protein expression probes, and the like, in applications such as the detection of biological compounds both in vitro and in vivo. Luminescent nanocrystals offer several advantages over conventional fluorophores, particularly for multiplexed and/or high sensitivity labeling. Nanocrystals typically have larger absorption cross sections than comparable organic dyes, higher quantum yields, better chemical and photochemical stability, narrower and more symmetric emission spectra, and a larger Stokes shift. Furthermore, the absorption and emission properties vary with the particle size and can be systematically tailored.
A variety of methods have been reported for the preparation of nanocrystals. These methods include inverse micelle preparations, arrested precipitation, aerosol processes, and sol-gel processes. A method commonly used for the preparation of binary nanocrystals is one in which an organometallic and elemental set of nanocrystal precursors is injected into a hot solvent as the solvent is being stirred. Product nucleation can begin immediately, but the injection causes a drop in the solvent temperature, which tends to halt the nucleation process. Nucleation and particle growth can be continued by heating the reaction mixture with further stirring, and the temperature can be dropped to stop the reaction when the desired particle size is obtained. As a result, the success of this batchwise “stirred-pot” method is strongly affected by system parameters such as the initial temperature of the solvent, the injection temperature and in particular the injection rate, the stirring efficiency, the concentrations of the reactant materials, the length of time that the mixture is held at the reaction temperature, and the efficiency of the cooling both after injection and after the desired endpoint is achieved. Some of these parameters are difficult to control with precision, and this can lead to poor reproducibility of the product. The lack of precise control also leads to nanocrystals with surfaces that are nonuniform, products that are readily degradable, and/or nanocrystals with low emission quantum yields.
The initial reaction conditions, i.e., the manner and conditions under which the reaction is initiated, are particularly important in controlling the quality and uniformity of the product, far more so than in other types of synthesis. Stirred-pot methods suffer in this regard since there are limits to how rapidly and uniformly the temperature of the reaction mixture can be changed or otherwise controlled. The temperature drop that occurs upon injection of the precursors will vary with the precursor temperature prior to injection, the volume of precursor injected and its rate of injection, the volume of the heated solvent, and the stirring efficiency. The difficulty in cooling rapidly when terminating the reaction often means that a lower reaction temperature must be used as a means of avoiding excess reaction. Further difficulties with stirred-pot methods are that they often involve the injection of large volumes of flammable or pyrophoric materials at very high temperatures, or the rapid evolution of gases, all of which present safety hazards.
Control of the properties of nanocrystals by the application of coatings or shells has been reported, notably in International Patent Publication No. WO 99/26299 (PCT/US98/23984), “Highly Luminescent Color-Selective Materials,” Massachusetts Institute of Technology, applicant, international publication date 27 May 1999, and references cited therein. The application of an inorganic shell, for example, can increase the quantum yield of the nanocrystal as well its chemical stability and photostability. The techniques for applying a shell are stirred-pot techniques that are usually similar to those used for the preparation of the core. Like the diameter of the core, the thickness of the shell affects the properties of the finished product, and the thickness will vary with the same system parameters that affect the core. The difficulties in controlling these parameters in a stirred-pot system lead to difficulties in controlling the nature and quality of the final product.